1. Field of the Invention
The present invention relates to the field of continuous foaming methods and apparatus, and more particularly to methods and apparatus for achieving flat top buns in continuous, open top foaming processes.
2. Prior Art
Flexible polyurethane foam is widely used for cushioning and sound absorption in a great number of commercial and consumer applications. By way of example seat cushions, mattresses and crash-padding commonly utilize flexible urethane foam with an appropriate funcional and decorative covering thereover of cloth, vinyl or other materials. While urethane foam may be and is molded in some form of closed mold for some applications, it is common to foam the constituents in an open top mold in a continuous process to form large buns, which are then sliced into sheets or blocks of the desired size. The equipment used in such processes comprises a long conveyor system onto which is disposed a disposable material such as a polyethylene or paper film, supported by a bottom conveyor and side conveyors (or fixed sides with a moveable film), to define a generally U-shaped trough for constraining the bottom and side surfaces of the foam. The foam reactants are mixed and dispensed at one end of the trough, which quickly foam as they progress through the machine to form a long continuous bun existing from the opposite end thereof.
Without more, the apparatus hereinbefore described results in a bun similar in cross section to a loaf of bread, characterized by a rounded top surface having a much greater height at the center thereof than at the sides. Such a bun shape is highly undesirable, as the slicing of the bun into rectangular sheets or blocks results in substantial waste caused by the curvature of the top surface. Accordingly various methods and apparatus have been devised which tend to provide a much flatter top surface of the bun without constraining the free rise of the foam.
It is well known that the rounded top surface of a freely rising bun is caused primarily by viscous effects between the rising foam material and the sidewalls defining the trough within which foaming is occurring. Accordingly, various techniques and apparatus have been devised for at least temporarily defining sidewalls effectively rising with the foam to negate the viscous effects adjacent the sidewall, and to encourage the free rise of the sides of the bun to the same extent as is encountered at the center of the bun. While a perfectly flat top bun may not be achieved, the irregularity (curvature) of the top surface of the bun may be grossly reduced by this technique, thereby minimizing resulting waste in the subsequent slicing of the bun.
The prior art relating to processes for making flat top buns is quite extensive, as the economic incentive for achieving the desired result has been long standing and great. Of the large volume of prior art, however, certain selected prior art patents are believed most relevant both because of their specific apparatus, and for some of them because of their known commercial use.
In U.S. Pat. No. 3,091,811, apparatus and methods for molding a foam material in continuous open top molding processes are disclosed. In one embodiment, crepe paper is formed into a U-shaped channel at each side of the molding conveyor and guided first substantially longitudinally to the vicinity of the cream line, and then upward at a predetermined angle in accordance with the rise and linear progression of the reactants. When the foam has nearly reached its full height, the strips of crepe paper continue upward and are drawn out of the conveying trough. In another embodiment, large discs supported on a common horizontal shaft are disposed at each side of the trough adjacent the cream line and caused to rotate, thereby defining an upward component at the sides of the trough during foaming. It will be noted in both of these embodiments that the item or material defining the upward component of velocity during foaming is contemplated as being removed from the trough as the foaming mass approaches its full rise, which would require a very heavy material to allow its being pulled away from the foam mass, and which normally would cause considerable scarring of the sides of the bun, thereby achieving the desired flat top characteristic at the expense of substantial waste at the sides of the bun. This patent, however, does disclose the basic concept of defining upward progressing sidewalls in the foaming region to achieve the desired result, though until now, the problem has remained as to how to achieve the result on a production basis in a simple, low-cost and highly reliable manner.
In Japanese Pat. No. 41-17667, rolls of suitable sheet material are disposed at each side of the trough, with material dispensed from the rolls being directed first horizontally and then upward along the desired trajectory along the conveyor. The horizontal component of the trajectory is defined, or more appropriately maintained, by toothed wheels adjacent the foam line rotating about substantially vertical axes and firmly engaging the material so as to limit its direction up to that point. Thereafter the material proceeds upward along a trajectory defined by a pair of rollers encouraging the material upward and on out of the foaming trough. The system contemplates a rather narrow strip of material being used at each side of the trough, which strip, as already mentioned, continues upward to be separated from the bun adjacent the region of near maximum rise thereof.
U.S. Pat. Nos. 3,719,734 and 3,751,197 disclose methods and apparatus to achieve flat top buns in a continuous process by the guiding of relatively wide plastic sheets longitudinally to the cream line, then upward in accordance with the foaming rate for release at a point adjacent the region of full expansion of the reactants. The system utilizes, adjacent each side wall, a continuous chain having a plurality of individual grippers to grip the plastic sheets adjacent the top thereof and to define the upward directed trajectory, resulting in a relatively mechanically complex system and requiring relatively thick plastic sheets or films so as to respond to the grippers without stretching or tearing.
U.S. Patent Nos. 3,809,512 and 3,812,227 also disclose methods and apparatus for continuous molding of flat top buns utilizing plastic sheets of relatively wide material having an upward trajectory in the foaming region. This system utilizes a fold-bar at the bottom of the trough somewhat downstream of the foam line over which a downward proceeding plastic film is directed so as to redirect the sheet along the desired upward trajectory defined by the pinch rollers engaging the top edge of the film adjacent the point of maximum foam rise. While the system appears relatively simple, control of the foldbar and the mechanism assuring proper rise of the plastic sheet along the trajectory are critical, and a relatively strong plastic sheet is required because of the local mechanical stresses created by the rollers defining the upward trajectory. In practice, tape reinforcement of the film edge is required. In this regard it should be noted that any system which does not gently but positively define the trajectory of the plastic sheet along the full length of the foaming region, but only defines that trajectory by rollers or other devices at the end points thereof, will necessarily require a much stronger plastic sheet because of the tension and concentration of stresses therein at the local points of support. Also, since the fold bar projects well beyond the cream line, scarring of the resulting bun may be caused thereby.
U.S. Pat. Nos. 3,768,937 and 3,875,276 disclose a method and apparatus for achieving the desired result using continuous tubular shaped plastic films at each side of the trough. The films are directed in a downward and forward trajectory off of rolls, with a guide plate inserted into each tubular member during the downward portion of the trajectory to maintain the basic tubular shape. The guide plates are folded about a horizontal line in the region of the cream line so as to define the desired upward directed trajectory starting approximately at the cream line. A cross section of each guide plate in the region of the fold is an approximate U-shape, with another plate extending downward between the legs of the U-shape to assure the desired contour and motion of the tubular member in that region. The tubular members proceed along the desired upward directed trajectory (defined by the guide plates) through the foaming region, with friction being minimized by plastic coatings on the guide plates and by the injection of air along the "leading edge" thereof. The tubular members are slit adjacent the region of maximum rise so as to freely proceed longitudinally with the cured bun outward through the machine. Adjustments are provided for the longitudinal position of the guide plates, though other parameters of the system appear to be nonadjustable. While this system has been used commercially, the friction encountered in the region of the fold of each guide plate generally requires the use of a relatively thick plastic film for the tubular members, and then the mechanical complexity and lack of adjustment therein prevents "fine tuning" of the system to achieve optimum results under varying conditions. Furthermore, the thickness of the assembly in the region of the foam line and for some distance thereafter, and perhaps other reasons, causes considerable scarring on the sides of the bun, thereby creating an additional source of waste.